Calcium Role in Gap Junction Channel Gating: Direct Electrostatic or Calmodulin-Mediated?

The chemical gating of gap junction channels is mediated by cytosolic calcium (Ca²⁺_i) at concentrations ([Ca²⁺]_i) ranging from high nanomolar (nM) to low micromolar (µM) range. Since the proteins of gap junctions, connexins/innexins, lack high-affinity Ca²⁺-binding sites, most likely gating is mediated by a Ca²⁺-binding protein, calmodulin (CaM) being the best candidate. Indeed, the role of Ca²⁺-CaM in gating is well supported by studies that have tested CaM blockers, CaM expression inhibition, testing of CaM mutants, co-localization of CaM and connexins, existence of CaM-binding sites in connexins/innexins, and expression of connexins (Cx) mutants, among others. Based on these data, since 2000, we have published a Ca²⁺-CaM-cork gating model. Despite convincing evidence for the Ca²⁺-CaM role in gating, a recent study has proposed an alternative gating model that would involve a direct electrostatic Ca²⁺-connexin interaction. However, this study, which tested the effect of unphysiologically high [Ca²⁺]_i on the structure of isolated junctions, reported that neither changes in the channel's pore diameter nor connexin conformational changes are present, in spite of exposure of isolated gap junctions to [Ca²⁺]_i as high at the 20 mM. In conclusion, data generated in the past four decades by multiple experimental approaches have clearly demonstrated the direct role of Ca²⁺-CaM in gap junction channel gating.